I uninstalled 10g database from my machine but the install/uninstall utility didn't delete the oracle home folder. I have been able to manually delete everything left behind, except the oci.dll in the bin folder. I get an error message from Windows that I don't have access or the file is in use. I'm logged in as Administrator.

Yes it works. I tried it.
I moved the files to desktop and renamed it. reboot my machine and moved it to recycle bin.
before that make sure you delete the oracle installation folder.
Clear the registries.
HKEY_CURRENT_USER\SOFTWARE\ORACLE
HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet Services\EventLog\Application\Oracle.oracle
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet Services\OracleDBConsole
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet Services\Oracle10g_home
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet Services\OraclService
Note that the services control panel will still show the old services until you reboot.
# Delete the Oracle home directory
# Delete the Oracle Program Files directory:
C:\Program Files\Oracle
# Delete the Oracle Start Menu shortcuts directory:
C:\Documents and Settings\All Users\Start Menu\Programs\Oracle*
Where * indicates the name of your install. Look for and remove all Oracle directories from that location.
# Remove Oracle refereces from the path. To edit your path go to:
Start->Settings->Control Panel->System->Advanced->Environment Variables

In this case I think
stopping the services ComSysApp (COM+ System Application) and Distributed Transaction Coordinator will help.

But in future, Install Process explorer from sysinternal tools(you can google this). This is Microsoft tool so you are totally safe to run this on your server. You can see which process or DLL is being held by which process and then you can take specific action according to the situation.
This will be useful for may other purposes also like tuning etc.